A theoretical description of atomic photoionization by attosecond pulses in the presence of an intense laser pulse is presented. It is based on the numerical solving of the non-stationary Schrodinger equation which includes on an equal footing the realistic atomic potential and the electric fields of both pulses. The calculated energy spectra and angular distributions of photoelectrons are compared with those obtained using a simple approximate model based on the strong- field approximation. The agreement is excellent for large energy of photoelectrons. When the energy is small, the rescattering of electrons by the ionic core affects the cross section considerably making the strong- field approximation inadequate. Influence of the electron orbital polarization on the ionization cross section is investigated.